The present invention relates to a signal encoding apparatus and a method therefor, a signal decoding apparatus and a method therefor, a program and a recording medium which are suitable when used in the case where time series signal limited to frequency band which is the encoding side is extended to signal in broader frequency band at the decoding side.
This Application claims priority of Japanese Patent Application No. 2002-241052, field on Aug. 21, 2002, the entirety of which is incorporated by reference herein.
In recent years, in efficient encoding of audio signal, mechanism of auditory sense of the human being is utilized to thereby have ability to compress data quantity having sound quality corresponding to CD (Compact Disk) into data quantity of about 1/10 of the original CD. At present, goods utilizing theses technologies are circulated also on the market, and it is realized that audio signals are recorded on smaller recording media, and/or are distributed (delivered) through network.
In such efficient compression, individual formats are respectively employed. If corresponding format is within the range of format, it is possible to freely control, to some degree, sound quality and bit rate at the encoding side. For example, also with respect to Mini Disk (MD) (Trade Mark by Sony Corporation), two modes of LP2 and LP4 using the same efficient compression technology exist as the long time recording mode. In the case of LP4, data is further compressed into one half with respect to LP2 to thereby permit recording time which is two times greater than that of LP2 while sound quality is inferior.
However, in such efficient compression technology, since design and standardization are made in the state where bit rate and sound quality are clearly determined as target, when bit rate is further lowered while maintaining standard (format), sound quality is extremely deteriorated. In order to avoid such circumstances, there is generally employed a method of performing improvement in efficient coding algorithm at the encoding side, or limiting signal in the high frequency band where auditory sense of human being is dull to distribute extra bits to signal in the lower frequency band.
Meanwhile, in the case where signal in the high frequency band is limited in order to maintain sound quality to lower bit rate while maintaining the format in a manner as described above, there is also a trial for reproducing signal in the high frequency band at the decoding side. For example, there are such a technology to double reproduction frequency band of PCM signal of 44.1 kHz sampling which is described in the Japanese Patent Publication Laid Open No. 1990-311006, and such a technology to extend frequency band of telephone at the receiving side which is described in the Japanese Patent Publication Laid open No. 1997-55778.
The technologies as stated above have the merit that change of format is not required so that it is sufficient to perform improvement only at the decoding side, but do not exhibit dramatic effect in point of sound quality because there is a necessity to extend the frequency band only from received signal, and are such that distortion from viewpoint of auditory sense is offensive to the ears in the high frequency band particularly in the case where there does not so exist correlation between low frequency band and high frequency band, etc. in dependency upon inputted sound source.
Moreover, in the above-described Japanese Patent Publication Laid Open No. 1997-55778, pitch analysis is performed at the time of decoding to add spectrum of frequency of multiple of n thereof to signal in the high frequency band to thereby extend frequency band of telephone at the receiving side. Since it is rare that plural pitches exist in the case of sound (speech) for telephone use, whereas there are many instances where plural pitches are included in the case of general audio signal, employment of such method is not effective, and there are many cases where pitch analysis does not function from the beginning.
On the other hand, in the case where format is extended so that band-limited reproduction is permitted to be performed in equipments which employ conventional format, and reproduction of high quality in which frequency band has been extended is permitted to be performed in equipments which employ new format, both change at the encoding side and that at the decoding side are required. In this case, result which is better than that by improvement only at the decoding side can be obtained. For example, in the technology for improving dynamic range and reproduction band of CD which is called HDCD, parameter such as filter kind, etc. for extending frequency band is hidden into the conventional format so that it is below audible level to thereby realize improvement in quality.
In addition, without being limited to such extension of format as described above, in mobile telephone and/or semiconductor recording equipment such as flash media, etc., it is desirable to have low bit rate and to have higher sound quality. Further improvement in performance by introducing new technology with respect to the existing Codec by waveform encoding is required.